Multipole permanent magnets, such as quadrupole magnets are used in a number of applications, including their use as a component in drift tubes incorporated within a charged particle beam accelerator. Recent advances in rare earth cobalt materials have created extremely strong permanent magnets, such as from samarium cobalt. Appropriate discussions concerning these magnets and the materials used may be found in U.S. Pat. No. 4,429,229, issued Jan. 31, 1984, to Gluckstern; and U.S. Pat. No. 4,355,236 issued Oct. 19, 1982, to Holsinger, et al. Within many linear accelerator drift tubes, the magnet is employed to focus the charged particle beam passing therethrough along the axis of the drift tube. The reader is referred to co-pending U.S. Pat. application 07/507,768, filed Apr. 12, 1990, with a common assignee, for a discussion of a drift tube assembly and its utilization in a linear beam accelerator.
Typically, a quadrupole permanent magnet is employed which has a hollowed cylindrical shape, the cylinder being formed from elongated magnetic material segments, each having an arcuate cross-section. In order to maintain a near ideally symmetrical quadrupole, it is important that the various segments be precisely and permanently positioned to form a perfect hollowed cylinder. The prior art frequently utilizes 16 segments in the construction of a magnet. In view of the fact that the various segments naturally tend to repel one another, it is difficult to assemble them. Further, unless they are securely symmetrically fastened to one another, they can incrementally shift, relative to one another, as a result of heat expansion and contraction over time. This, of course, diminishes the symmetrical magnetic quadrupole effect that is necessary.
Metallurgical bonding of the individual segments has been previously attempted by electronic beam welding and brazing but these methods are impractical due to the fact that a rare earth quadrupole permanent magnet may be adversely affected if subjected to temperatures well over 100.degree. for any protracted period of time. Attempts to use normal solder to achieve metallurgical bonding have met with failure due to the fact that the rare earth-cobalt magnetic material is brittle and has a low coefficient of thermal expansion. Thus, when the finished product is exposed to wide thermal variations during operation, the magnetic material fails at the interfaces between adjacent segments.
The prior art has attempted to solve this problem by encircling the magnet segments with a hoop for exerting radially inwardly directed forces which wedge the individual magnet segments together. A most common material for such a hoop is aluminum. However, the marked difference in the thermal coefficient of expansion between aluminum and the rare earth-cobalt material is substantial enough to cause different rates of expansion and contraction when thermal cycling occurs over a relatively large temperature range. This results in induced slipping forces exerted against the segments which adversely affects the desirable symmetrical quadrupole magnetic pattern.